# 4.3 How Atoms Differ Atomic Number - Atomic Number = # protons in an atom - # of protons determines kind of atom (atoms of an element always have the.

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4.3 How Atoms Differ

Atomic Number - Atomic Number = # protons in an atom - # of protons determines kind of atom (atoms of an element always have the SAME number of protons) - The atomic number = the number of electrons in a neutral atom - Written just above the atomic symbol on the periodic table

Complete the Table: ElementAtomic ## Protons# Electrons 6 N Pb 3 19 Br

ElementAtomic ## Protons# Electrons C666 N777 Pb82 Li333 K19 Br35

Isotopes All atoms in an element have the same number of protons The number of neutrons vary Atoms of an element with different numbers of neutrons = ISOTOPES

Isotopes Elements in nature = a mix of isotopes The abundances of different isotopes are constant EX: in ANY banana, 93% K has 20 neutrons, 7% have 22 neutrons http://www.youtube.com/watch?v=Jdtt3LsodAQ&feature=related

Mass of atoms One atomic mass unit (amu) is defined as 1/12th the mass of a carbon-12 atom.atomic mass unit One amu is nearly, but not exactly, equal to one proton or one neutron.

Mass of subatomic particles

Mass Number # PROTONS + # NEUTRONS = MASS NUMBER EXAMPLE: What is the mass number for manganese if there are 30 neutrons in the nucleus?

Element Symbols Top is mass number Top is mass number Bottom is atomic number Bottom is atomic number Practice Practice

Symbols EXAMPLE 1: Find the –number of protons –number of neutrons –number of electrons –Atomic number –Mass Number F 19 9

Answer… - - 9 protons - - 10 neutrons - - 9 electrons - - Atomic # = 9 - - Mass # = 19 - - What would the mass number be of a fluorine isotope with two additional neutrons?

Symbols EXAMPLE: Find the –number –number of protons of neutrons of electrons –Atomic –Atomic number –Mass –Mass Number Br 80 35

Answer –Number of protons: 35 –Number of neutrons: 45 –Number of electrons: 35 –Atomic number: 35 –Mass Number: 80

Symbols If an element has an atomic number of 34, a mass number of 78, find… –number of protons –number of neutrons –number of electrons And WRITE the Complete symbol

Answer… - - 34 protons - - 34 electrons - - 44 neutrons - - Symbol: Se 78 34

Symbols If an element has 91 protons and 140 neutrons what is the……..? –Atomic number –Mass number –number of electrons –Complete symbol

Pa 91 231

Symbols If an element has 78 electrons and 117 neutrons what is the ………? –Atomic number –Mass number –number of protons –Complete symbol

Pt 78 195

Naming Isotopes Put the mass number after the name of the element Put the mass number after the name of the element carbon- 12 carbon- 12 carbon -14 carbon -14 uranium-235 uranium-235

Mass of the atom Atomic mass on the periodic table is NOT an even # The atomic mass of an element is the weighted average mass of the isotopes of that elementatomic mass Depends on abundance of each isotope

Atomic Mass - How heavy is an atom of oxygen? - The MASS NUMBERS we’ve been using are measured in amu - 1 amu (atomic mass units) is ABOUT the mass of 1 proton or 1 neutron (or 1/12 of a carbon -12 atom) - On the periodic table, the MASS of each element is NOT reported as a whole number… - We can’t have HALF a proton or HALF a neutron in an atom… so why aren’t these masses whole numbers??

Atomic Mass - - Chlorine-35 is MORE common in nature than chlorine-37…so the mass of 1000 Cl atoms will be CLOSER to 35000 amu - - To take the average of the two isotopes, we need to use the WEIGHTED AVERAGE: - - In nature, 75.53% of Cl atoms are chlorine-35, and 24.47 % are chlorine- 37… - - (% is called % abundance)

Calculating the WEIGHTED AVERAGE - - Write each percent as a decimal - - Multiply that decimal by the isotope’s mass - - Find the average by adding together Average = % as decimal x mass + % as decimal x mass - Average Mass of a Chlorine atom: (0.7553)(35) + (0.2447)(37) = 35.4894 amu (0.7553)(35) + (0.2447)(37) = 35.4894 amu % abundance isotope 1 Mass isotope 1 % abundance isotope 2 Mass isotope 2

Atomic Mass: EXAMPLE - Calculate the atomic mass of copper if copper has two isotopes. 69.1% have a mass of 62.93 amu and the rest have a mass of 64.93 amu…

ANSWER… Mass of Cu = (.691)(62.93) + (.309)(64.93) = 63.548 amu

Atomic Mass: EXAMPLE 2 - Magnesium has three isotopes. 78.99% magnesium-24, 10.00% magnesium-25, and the rest is magnesium-26 amu. What is the atomic mass of magnesium?

Answer… Mass of Magnesium = 7899 x 24) + (.1000 x 25) + (.1101 x 26) = (. 7899 x 24) + (.1000 x 25) + (.1101 x 26) = 24.3202 amu

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